Recently PГ©rez-Losada et al (1999) described highly significant subpopulation structuring and clinal allozyme frequency changes between Atlantic and Mediterranean samples of the cuttlefish Sepia officinalis around the Iberian peninsula, which they explained on the basis of the secondary intergradation model described above.

Nevertheless, the possibility of such geographically based genetic patterns resulting from adaptive responses to different environmental pressures (natural selection) or due to isolation by distance among populations, cannot presently be excluded. Two factors suggested to account for the contemporary position and maintenance of discontinuities in frequencies of genetic markers in populations in this area are one-way surface current flows through the Strait of Gibraltar (eg, allozymes in hake вЂ“ RoldГЎn et al, 1998) and the presence of a well defined hydrographic boundary of surface waters between AlmerГa (SE Spain) and Oran (Algeria) ( Tintore et al, 1988), the AlmerГa-Oran oceanographic front (eg, allozymes and mtDNA in mussels вЂ“ Quesada et al, 1995 Sanjuan et al, 1996 microsatellites in fish вЂ“ Naciri et al, 1999). Genetic divergence of populations during isolation followed by secondary contact after the re-opening of the Strait of Gibraltar, and with currently restricted gene flow between the geographically divergent populations (ie, secondary intergradation), are predicted to have produced present patterns in genetic structuring. To explain the concordance of patterns of genetic differentiation observed in these marine species, a hypothesis has been proposed wherein Mediterranean and Atlantic populations have been isolated due to sea level changes during glacial maxima and subsequently reconnected, possibly several times, in the past. Studies on several fishes, molluscs and crustaceans (see Sanjuan et al, 1996, 1997 and references therein RoldГЎn et al, 1998 Naciri et al, 1999 Zane et al, 2000) have suggested the importance of the junction of the Atlantic Ocean and Mediterranean Sea in phylogeographical structuring of marine species.

A pronounced вЂstepвЂ™ change between SW Mediterranean samples associated with the AlmerГa-Oran front suggests this oceanographic feature may represent a contemporary barrier to gene flow. Distinct and significant clinal changes in allele frequencies between Atlantic and Mediterranean samples found at five out of seven loci, however indicate these results might be also consistent with an alternative model of secondary contact and introgression between previously isolated and divergent populations, as previously proposed for other marine species from the Atlantic-Mediterranean area. Microsatellites detected highly significant subpopulation structuring ( F ST= 0.061 R ST = 0.104), consistent with an isolation-by-distance model of low levels of gene flow. Genetic variation at seven microsatellite loci screened in six samples from NE Atlantic and Mediterranean coasts of the Iberian Peninsula was high (mean Na = 9.6, mean H e = 0.725). This study presents microsatellite DNA data for a mobile invertebrate species in this area, the cuttlefish Sepia officinalis, allowing comparison of this relatively new class of DNA marker with previous allozyme results, and examination of the relative effects on gene flow of the Strait of Gibraltar and the AlmerГa-Oran oceanographic front. The Atlantic Ocean-Mediterranean Sea junction has been proposed as an important phylogeographical area on the basis of concordance in genetic patterns observed at allozyme, mtDNA and microsatellite DNA markers in several marine species.

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